Primary hyperrapathyroidism is a common endocrine disorder of undertain cause and pathogenesis. The crucial factors that contribute to the hypersecretion of parathyroid hormone (PTH) and hypercalcemia in this disorder are insensitivity to suppression of PTH secretion by high extracellular (EC) Ca++ concentrations and increased parathyroid tissue mass. The regulation of PTH secretion by Ca++ differs strikingly from stimulus-secretion coupling in other exocytotic systems. Hypocalcemia maximally stimulates while hypercalcemia inhibits PTH release. High EC Ca++ concentrations and Ca++ ionophores increase intracellular (IC) free Ca++ in vitro but they suppress PTH release. These observations support the idea that high IC Ca++ inhibits, rather than stimulates, PTH release. The intracellular mediators that transmit the hypocalcemic signal for maximal PTH release have not been identified. The clue that activation of protein kinase C or C-kinase and stimulation of phosphoinositide turnover may modulate PTH release is supported by the finding that phorbol esters reverse the inhibitory effects of high EC Ca++ on PTH release. We propose to test the hypothesis: that low Ca++-stimulated PTH release involves activation of C-kinase and that enhanced phosphoinositide turnover and conversely, inhibition of C-kinase activity contribute to the EC Ca++-induced rise in IC Ca++ and suppression of PTH release in parathyroid cells. To test this hypothesis, we will measure C-kinase activity and the breakdown products of phosphoinositide hydrolysis -- diacylglycerol and the inositol-phosphates -- at different EC Ca++ concentrations. We will then determine whether GTP binding to a guanyl nucleotide-binding protein couples changes in EC Ca++ to increases in IC free Ca++. We will test this idea by measuring IC free Ca++ and PTH release after incorporation of GTP or a stable GTP analog into intact parathyroid cells or after pretreatment with pertussis toxin, an irreversible inhibitor of certain GTP binding proteins. The observation that growth factors and mitogens stimulate protein kinase C activity and phosphoinositide breakdown offers a potential link between the regulation of PTH release and of parathyoid cellular proliferation. Thus, we will test the effects of epidermal growth factor and platelet-derived growth factor on both proliferation and the regulation of PTH release by EC Ca++. These studies should identify potential cellular defects that might contribute to the disordered secretion and cell growth characteristic of hyperparathyroidism.